Thymus extract was injected daily in large amounts into male guinea pigs for 5-110 days. In the circulating blood and in the spleen, lymphocytes increased and eosinophils decreased. In the bone marrow, both increased. The relation between young and mature eosinophils suggested that this increase might result from a decreased output rather than from an increased production. Histological examination of the thymus showed it to be in a resting condition. Since lymphocytes increased everywhere except in the thymus, it was suggested that thymus extract injections produced a decreased rate of hormone production in the thymus.